Electroluminescent displays (ELDs) are conventional light-emitting electronic display devices ELDs include components such as inorganic electroluminescence elements and organic electroluminescence elements (hereinafter, also referred to as organic EL elements). Inorganic electroluminescence elements, which have been used as planar light source, require high AC voltage to be driven.
An organic EL element usually has a luminous layer containing a luminescent compound disposed between a cathode and an anode. In the organic EL element, electrons and holes are injected into the luminous layer to recombine therein, which generates excitons. The excitons are deactivated while emitting light. The organic EL element emits light (fluorescence or phosphorescence) in such a manner. Organic EL elements can emit light at a low voltage in the range of several volts to several tens of volts. Organic EL elements also provide a wide viewing angle and high visibility due to their self-luminescent characteristics. Organic EL elements are also attracting attention from the viewpoints of space saving and portability because they are full solid state thin-film element.
Since an organic EL element based on phosphorescence from the excited triplet had been reported by Princeton University, phosphorescent materials emitting at room temperature (25° C.) have been intensively studied toward practical use of organic EL elements.
The recently discovered organic EL elements based on phosphorescence have luminous efficiency theoretically raised by approximately four times in comparison with a conventional element based on fluorescence. Therefore, materials, layer structures and electrodes for light-emitting elements are researched and developed worldwide.
For example, many compounds, primarily heavy metal complexes such as iridium complexes, have been synthesized and used for luminous layers of organic electroluminescence elements (also referred to as organic EL elements).
Although an organic EL element based on phosphorescence is a system of great potential, major technical issues for the element are the way of controlling the position of the emissive center, particularly stable recombination inside the luminous layer and stable emission of light, as well as enhancement of the luminescent property of a phosphorescent dopant itself, from the viewpoints of efficiency and lifetime of the element.
In order to enhance the luminescent property of a phosphorescent dopant, there are two possible approaches: (1) an increase in the radiative rate constant (kr) and (2) a decrease in the non-radiative rate constant (knr), when the lowest excited triplet state (T1) is deactivated to the ground state (S0).
A possible specific measure for decreasing the non-radiative rate constant (knr) is steric control of the structure of a ligand of the phosphorescent dopant to reduce the structural change between the ground state and the excited state. With regard to the iridium complex, which is a typical phosphorescent dopant, one of examples of such a measure is control of the steric structure by a combined ligand of dibenzofuran and pyridine as disclosed in PTL 1, for example.
Similar applications are found on iridium complexes formed with phenylpyrazole derivatives (see PTL 2), phenylimidazole derivatives (see PTL 3), and derivatives containing a carbene moiety as a ligand (see NPL 1), and on a platinum complex (NPL 2). These complexes have a low reorganization energy, due to decreased structural changes between the ground state and the excited triplet state.
From the viewpoint of decreasing the reorganization energy to decrease the non-radiative rate constant (knr), extension of the conjugated system from naphthalene ring to pentacene ring is known to decrease a reorganization energy (see NPL 3). Such a method is based on the effect of delocalized electrons.
Regarding the reorganization energy of luminescent hosts, disclosed is the use of a host compound having a reorganization energy level of 0 eV to 0.50 eV when the host compound is converted into anionic radicals, which provides organic EL elements with improved properties, such as an increased luminance and a prolonged lifetime (PTLs 6 and 7).
Durability of a phosphorescent dopant varies widely depending not only on the luminescent dopant but also on a host compound used in combination with the luminescent dopant. Since the interaction between the host compound and the luminescent dopant in a film influences significantly on carrier mobility and the durability, the combination of the host compound and luminescent dopant may be an important factor for improved durability. Disclosed is a technique of enhancing luminous efficiency and heat resistance of elements with a host compound having a specific heterocyclic structure in combination with a luminescent dopant (PTLs 4 and 5).
Regarding the luminescent dopant, disclosed is use of a compound having a difference of 0 nm or more and 5 nm or less between a maximum emission wavelength on a shortest wavelength side in an emission spectrum measured at 300 K and that measured at 77 K, which provides organic EL elements with improved properties, such as enhanced luminous efficiency and a prolonged lifetime (PTL 8).
Unfortunately, these techniques are not satisfactory in terms of providing an organic EL element that has high luminous efficiency and low driving voltage, excels in heat resistance and raw storability, and has a long lifetime. A further solution is therefore being sought for a lower non-radiative rate constant (knr) of the luminescent dopant to enhance the luminous efficiency of the element, for combination of optimal host compound and luminescent compound to increase the durability of the element, and for achieving such improvements at the same time.